A method for a base station (BS) operating in a wireless communication system, the method includes transmitting, to a user equipment (UE), first information indicating a threshold value for an uplink data split operation; and receiving a buffer status report (BSR) from the UE, wherein a value of a buffer size in the BSR is determined based on second information that indicates an amount of data available for transmission for a packet data convergence protocol (PDCP) entity of the UE when the amount of the data available for transmission is larger than the threshold value.